Conventional distributed multi-resonator radio frequency and microwave signal filters commonly include a plurality of resonators. These resonators typically comprise quarter wavelength or capacitively foreshortened quarter wavelength coaxial or helical resonator bars. The resonators are arranged in a conductive enclosure and are typically coupled to one another by capacitive or inductive means. Iris coupling is often used as a means of adjusting the coupling between the resonators. In these types of multi-resonator filters, tuning screws are commonly required to precisely adjust the frequency of the resonators. The tuning screws can be easily misadjusted, and thus, these types of filters are particularly prone to detuning.
The mechanical problems associated with the tuning screws have been largely solved by ceramic filter technology. Using ceramic filter technology, multi-resonator filters can be constructed in a single dielectric block. The filter typically includes a plurality of metallic cylindrical resonators consisting of metalized holes coupled to one another via the dielectric material. Coupling between the resonators may be adjusted by including decoupling irises or slots, or by varying the size and location of additional metalized or unmetalized holes in the dielectric block. U.S. Pat. Nos. 4,426,631, 4,431,977, and 4,462,098 illustrate exemplary single block ceramic filter designs having multiple inputs and multiple outputs.
Simple multiplexing of ceramic filters in a single block has been shown in U.S. Pat. Nos. 4,431,977 and 4,742,562. However, there are many important applications where it is desirable to provide a common input and a common output for more than one frequency band. For example, a global positioning satellite receiver can require two separate frequency bands from a single output, such as an antenna, so that it may filter the undesired frequencies and then amplify the resulting output signal with a common low noise amplifier. To accomplish this, a dual passband filter without passive dividers or combiners is required. The external transmission lines disclosed in U.S. Pat. No. 4,431,977 may be used for this purpose. However, they are lossy, and can be larger than the filter itself. Further, the external transmission lines must be attached to the filters as a manual process.